One of the most popular uses of silicon micromachined sensing technology has been in the field of pressure sensing. Here, whether in the automotive, industrial control, or medical marketplaces, piezoresistive sensing technologies have solved many vital pressure sensing problems as well as created new applications.
One popular type of pressure sensor is mounted to a pipe-thread so that it can be easily fastened into a matching threaded fitting. Because these sensors are capable of being mounted in any standard pipe fitting (typically 1/8 or 1/4 inch NPT), they can mate directly to the pressure source or pressure-carrying line or pipe to directly measure the pressure of a media. The media to be measured can be a gas such as air, natural gas, oxygen, nitrogen or any other gaseous composition. In addition, the measured media can be a pressurized liquid such as water, oil, gasoline or any other liquid solution. The pipe-threaded form factor has led to a group of products assuming a similar design and construction. These sensors are typically constructed of stainless steel so that they can withstand high media pressures without cracking or leaking when subjected to high pressures.
In conventional pipe-fitted sensors, one end is normally a hollow, cylindrical, threaded tube, that is adapted to mount into a threaded pipe fitting. The other end of the sensor is normally an enclosed cylindrical tube that houses the silicon pressure sensor and its substrate. Separating the sensor from the media, whose pressure is to be measured, is normally a flexible stainless steel membrane bound to the inside of the stainless steel tube, near the sensing element. An oil is typically placed in the region between the flexible membrane and the sensor so that the pressure of the media is transferred through the oil to the sensing element. In this manner, the sensor device is physically separated from the media being sensed under pressure.
Although this construction provides several advantages, many disadvantages are also present. While conventional stainless steel pressure sensor bodies are normally strong enough to withstand pressures of approximately 5-10,000 PSI, they are also expensive to manufacture. Thus, the cost of manufacturing a stainless steel pressure sensor is higher than it might be if the body was made of a less expensive material.
Another facet of modern electronic pressure sensors is their mechanism for communicating with other devices. A typical sensor either displays information, or transmits information to an external device, such as a computer. The output of the sensing element is processed into a particular format for communicating with external devices whether or not the sensing element is based on a capacitive or piezoresistive element. Typically, the weak signals generated upon detection of pressure changes on the sensing element are amplified before being transmitted to an external device. Many vendors have provided various systems for amplifying signals generated from the sensor. Generally, however, the sensor outputs can be categorized by three typical output formats:
1. Unamplified--The unamplified signal is transmitted to another assembly which processes the signals (e.g. 0-50, 0-100 mV Out) PA1 2. Voltage Out--A voltage is generated which is an amplified output of the basic sensing assembly (e.g. 0-5, 1-6 Volts Out) PA1 3. Current Out--A current is generated which is proportional to the output signal of the sensing element. (e.g. 4-20 ma Out)
However, because each of these sensors produce an analog output, if digital processing is required by the external device, at least one further analog to digital conversion is required. Performing this conversion is disadvantageous because it adds complexity and cost to the sensor design.
As with many silicon micromachined sensors, detection characteristics of the sensing element and the other electrical components mounted in its assembly vary with temperature. Although temperature variations can cause both sensitivity and offset changes, they can easily be measured, and compensated for during thermal testing of the sensor. Typically, the problems associated with these offset and sensitivity changes have been addressed in two ways. First, laser trimming compensation resistors have been used in the past to compensate offset and sensitivity variations of the bridge with temperature. This is achieved by trimming (using a laser, for example) a set of resistors which can be attached to various points of the bridge, which over temperature, will compensate the change of resistance of the silicon resistors of the piezoresistive bridges. This process requires temperature cycling the sensor, then trimming those resistors to counteract the temperature coefficients of the silicon resistors which make up the piezoresistive bridge.
The second technique is a more recent approach which utilizes digital and analog circuit techniques to modify the output signals according to the temperature characteristics of the specific sensor. These techniques have been incorporated in current Application Specific Integrated Circuits (ASICs) where temperature coefficients are stored digitally then input to analog circuits which provide offsets and gain adjustments when temperature changes are detected by the sensing element.
While the previously described technology has provided the marketplace with some solutions to their pressure sensing needs, significant disadvantages remain for many applications. For low cost automation applications, an integrated sensor solution should be inexpensive, easy to manufacture and test, provide direct digital outputs for direct computer interfacing and be field programmable or reprogrammable for flexibility in the processing network.
The stainless steel-signal conditioned sensors in the marketplace do not meet these requirements. First, manufacturing sensors with stainless steel body and welded stainless membranes remains a costly process. Testing products that incorporate temperature compensation is complicated due to the complexity of laser trimming and its inherent yield issues. Those sensors which use Application Specific Integrated Circuits (ASICs) to adjust pressure offsets and gains as a function of temperature using internal digital to analog circuitry can require an external programmer to modify the results or reprogram an internal memory. Most commercially available ASICs generate an analog signal, while the laser trimmed devices require separate analog amplification. To generate digital signals, both types of units still require a digital to analog converter.
Thus, what is needed in the art is a sensor device that can withstand high pressure media, while being economic to produce. In addition, the art needs an electronic sensor that can directly provide a temperature compensated digital output to external devices. The present invention provides such a system.